SMTP

SMTP ports you need to understand are:

  • TCP 25: default for non-encrypted communications.

  • TCP 465: port registered by IANA for SMTP over SSL (SMTPS). Note, this has been deprecated in favour of STARTTLS.

  • TCP 587: Secure SMTP (SSMTP) for encrypted communication using STARTTLS.

  • TCP 110: default port for POP3 non-encrypted communications.

  • TCP 995: default port for POP3 encrypted communications.

  • TCP 143: default port for IMAP in non-encrypted communications.

  • TCP 993: default port for IMAP in SSL/TLS communications.

Some useful SMTP commands for validation are:

  • HELO: initiate SMTP conversation with mail server.

  • EHLO: initiate ESMTP conversation with mail server.

  • STARTTLS: start a TLS connection to mail server.

  • RCPT: denote mail address of recipient.

  • DATA: initiate transfer of contents of an email message.

  • RSET: cancel an email transaction.

  • MAIL: used to denote mail address of sender.

  • QUIT: used to close connection.

  • HELP: display help.

  • AUTH: authenticate client to server.

  • VRFY: used to verify if a user's mailbox exists.

  • EXPN: request, or expand a mailing list.

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